The invention relates to a radio broadcasting system with transmitter identification wherein, at the transmitter side, a multiplex signal, which is frequency modulated on a main carrier, is transmitted, said multiplex signal comprising: an audio frequency information signal, in the case of stereo transmission a stereo information signal which is modulated on a suppressed stereo sub-carrier, a stereo pilot whose frequency is located between the frequency spectra of the audio frequency information signal and of the modulated stereo information signal and which serves, in a receiver, for demodulating the stereo information signal, as well as a binary transmitter identification signal modulated on a further sub-carrier, the frequency of this signal being located above the frequency spectrum of the modulated stereo information signal. In addition the invention relates to a transmitter for transmitting signals in accordance with such a system as well as to a receiver for receiving such signals.
When tuning the present FM radio receivers, the user often experiences great difficulties because the tuning scale only displays frequencies and/or channel numbers and not the names of the stations. In addition, one given program is often transmitted by several transmitters so that the user does not know if he has tuned to the strongest transmitter.
In order to provide the user with an easily recognisable identification of the FM transmitters and/or of the nature of the program transmitted by the transmitter, a radio broadcasting system with transmitter identification as described in the preamble has already been suggested to the CCIR (Comite Consultatif International des Radiocommunications). In this system, the transmitter identification signal is transmitted by means of a suitable carrier above the frequency spectrum of the stereo information signal. This carrier is frequency-modulated with the binary transmitter identification signal which, by means of a digital code, contains information on, for example, the name of the program, the location of the transmitter, the nature of the program and the channel number, so that, for example, the following information, consisting of 16 characters, is received: ##STR1##
The receivers for such a system are provided with a decoder which decodes the binary transmitter identification signal from the signal received and uses it, for example, for wholly or partly optically displaying the information thus transmitted, so that the user can immediately see to which transmitter his receiver is tuned. Alternatively, it is possible to arrange the receiver in such a way that, at the reception of a preset transmitter identification code, a portion of the receiver, or of a tape recording or reproducing apparatus, is switched on or off. In particular, if the transmitter identification signal contains a special code which is transmitted for traffic reports, this code can be used to switch on the reproducing section of a car radio receiver or to stop a tape reproducing device which is in operation.
The above-mentioned prior art transmitter identification system has been tested in practice with the following values:
The sub-carrier frequency was 66 KHz and the frequency sweep 1 KHz, so that, due to the binary information, the frequency was switched between 65 KHz and 67 KHz. (Frequency shift keying).
The code used was the 6-bit ASCII-code having 16 characters per information.
The amplitude of this transmitter identification signal was chosen such that, of the totally available frequency sweep of 75 KHz for the FM modulation of the main carrier, 1 KHz, i.e. 1.33%, was occupied by the transmitter identification signal. The comparatively small amplitude (1 KHz) of this signal is opted for because experiments proved that a greater amplitude may cause interference noise in some types of FM-receiver.
It appeared, however, that the necessarily small amplitude of the transmitter identification signal and the comparatively high frequency thereof (66 KHz) resulted in a poor signal-to-noise ratio. In order to recover the code signal flawlessly, the receiver requires a high-grade filter having a good quality factor and a good temperature stability. In addition, it appeared that, in spite of the use of such a high-grade and expensive filter, decoding of the transmitter identification signal no longer occurs flawlessly with aerial voltages below 10 .mu.V (at 60 ohm) whereas the average FM-receivers still furnish an acceptable mono-reception at such aerial voltages.